I just had a Trapper game. N2 I placed my Trap on WW, N3 it was triggered and attacked someone but no role appeared so I knew it was triggered by a Necromancer using his ghoul.
Mafia had no Hypnotist and even if Hypno can make it seem like it triggered he had to know I was trapper without consigs nor me claiming.
So I think because it was ONLY triggered by his ghoul, not by him using a dead town, it bugged out. I did not notice until N5 after lynching Necro and telling trans I'd trap him against the WW so he shouldn't trans that I could not place it down because it was still placed.
And N6, before WW killed me, I also noticed that I couldn't even take it down.
Here is an imgur link containing screenshots of N3 and N4. I am using the Browser Version of the game.
https://imgur.com/a/WTAjouK